Meter test switches, such as watthour meter test switches, are well known in the electric utility industry. Such switches operate in conjunction with other test instruments, such as ammeters, which must be inserted into the watthour meter circuit without interrupting current flow. Various switch designs have been proposed by different manufacturers. These designs, however, have certain undesirable features which the present invention eliminates.
Existing test switches of the type used for measuring current in a transformer rated kilowatthour meter use knife blade type switch sections for opening and bypassing circuits. Three different types of sections are used to provide the required test functions. These sections are generally referred to as potential (P), current test (C) and current shunt (C+) switches, like terminology being adopted hereinafter.
The P switch is an SPST switch having a threaded terminal post electrically connected to a yoke on which the knife blade pivots. To conduct current, the blade is pivoted to a position where it slides between a laterally expanding pressure contact which is electrically connected to a second threaded terminal post. The C+ switch is an SPDT switch having an electrical contact configuration similar to the P switch and a laterally expanding yoke provided with a retaining finger for limiting upward movement of the blade. The yoke of the C+ switch is connected electrically to the C switch so that current is shunted from the C+ switch to the C switch when the blade of the C+ switch is in the partially raised position contacting the yoke. The blade of the C+ switch, however, does not contact the yoke in its normally conductive position. The C switch is also similar to the P switch with the addition of a shunt connection from one of its terminals to the yoke of the C+ switch and abutting laterally expanding spring-type contacts for the insertion of a two-conductor test instrument probe. The spring contacts are so situated that the blade of the C switch does not make contact with them in any position.
Sets of the above described switches are mounted conventionally on a non-conducting base which is covered by a plastic cover when the switches are not in the "test" position. The three types of switches, P, C and C+, can be arranged in any configuration so as to satisfy the immediate needs of the user for a specific application. For example, three sets of the above-described switches and a neutral strap can be arranged on a base to measure current in a kilowatthour meter for a three-phase electrical system. However, once mounted on the base, the switches cannot be easily interchanged.
In any application, the knife blade design of the conventional test switch requires the use of multiple laterally expanding pressure connections. Due to the frequency with which the switches are opened and closed in normal use, the resistance of the contacts can vary considerably. This may result in incorrect metering or excessive heating, which could ultimately lead to destruction of the switch. When the switch is in use, with the plastic cover removed, the switch is exposed directly to the environment posing a potential hazard to the user.
The test switches suffer from other deficiencies as well. For example, the switch contacts must exhibit an extremely low, but relatively constant resistance. Yet, due to manufacturing tolerances in aligning the contacts, varying resistances often result. The resistance of each switch may also vary due to exposure of the switch to the environment. Tampering with the switches can also occur since the cover is easily removable. For example, an insulating tape can be placed over the knife blade to prevent electrical connection. This would result in an incorrect and/or lower watt hour meter reading. The current transformer core can become saturated. When saturated the voltage in the transformer coil is greatly increased which could cause the insulation in the core to break down and disintegrate causing the coil to short circuit or burn out.
Any combination of the above factors could also cause excessive heating of the switch which could warp or destroy such switch. Since the switches are mounted on a molded insulating type base, where the base constitutes the only support and alignment for the switch elements, replacement of a single switch due to malfunction or destruction while the switch is in use is impracticable without replacing the entire switch assembly.
The heating of the switch elements due to the variances in resistance can also affect the base and cause it to warp or fracture when bolted to a flat surface such as a metal plate thus requiring the replacement of the entire assembly.
The plastic cover acts as a locking mechanism for the switches because it can only be mounted on the base when all switches are in their "normal" or conducting position. This provides a check for returning the switches to their conducting position. However, the cover can be lost or damaged due to its detachability and no longer provide this check. Also, in the "test" mode, the cover is removed exposing personnel to high voltages across the contacts of the switches.
The test switches currently in use are operated in such a manner as to temporarily short-out a current transformer secondary winding so as to prevent damage to the current transformer while a calibrated current is inserted from test sources into the watthour meter current circuit. In addition the switches can be operated so that the current to the kilowatthour meter can be sampled by the insertion of a test probe into the meter test switch circuit without the necessity of breaking the circuit for each test.